1. Field of the Invention
The present invention relates generally to an anti-lock brake control system and method, and, more particularly, to an anti-lock brake control system and method which independently controls the brake pressures to the right and left front wheels, and commonly controls the brake pressures to the right and left rear wheels on the basis of the lower one of the wheel speeds of the right and left rear wheels when the front wheels are in a yawing moment control mode and when the lower speed rear wheel is in an anti-lock brake control mode, the brake pressure applied to the higher speed rear wheel is decreased at a smaller decreasing rate than the brake pressure applied to the lower speed rear wheel.
2. Related Art
While a vehicle is running on a slippery road such as a frozen road, the wheels may become locked when a brake is applied since the friction force acting between the road surface and the wheel tires is small. In a known anti-lock control system, as means for preventing this lock state, the braking forces are controlled such that a slippage factor of the wheel against the road surface is maintained at a predetermined value.
When a vehicle equipped with the anti-lock control system runs on a road, and only one of the right and left wheels runs on a road surface having low friction coefficient (referred to as ".mu. split road"), such as a frozen road surface, if the brakes for the right and left wheels are independently controlled, the braking force applied to the wheel on the road surface having high friction coefficient (referred to as "high-.mu. road surface") becomes larger than that applied to the wheel on the low-.mu. road surface. In an extreme case, the vehicle may spin. Particularly in the short-wheel base vehicles, high gravity-center vehicles, and vehicles of large scrab radius, it is likely that spin state will occur.
To cope with this problem, the present inventor has previously proposed another anti-lock control system as disclosed in, for example, commonly assigned U.S. Pat. No. 5,286,098. In the anti-lock control system of this patent, there exists a select low mode in which the braking forces applied to the right and left wheels are controlled on the basis of the lower one of the wheel speeds of those wheels, that select low mode being employed for the brake control of that right and left front wheels. In the anti-lock control system, when the front wheel on the low-.mu. road surface are locked, a brake hydraulic pressure applied to the front wheel on the high-.mu. road surface is decreased to the brake hydraulic pressure to the front wheel on the low-.mu. road surface so that a yaw moment control is accomplished.
However, the anti-lock control system in which the yawing moment control is applied to only the front wheels has the following problem. In the yawing moment control mode, the wheel on the high-.mu. road surface cannot effectively utilize the friction of the road surface. Further, the brake hydraulic pressures applied to the right and left rear wheels are decreased at the same pressure decreasing rate on the basis of the select low control. Therefore, the rear wheel on the high-.mu. road surface also insufficiently utilizes the friction of the road surface. In other words, the braking force applied to the rear wheel on the high-.mu. road surface cannot be effectively utilized. As a result, the braking distance must be undesirably long.